In all internal combustion (i.c.) engines the so-called air/fuel ratio (A/F) is of great importance to the function of the engine. To obtain a proper combination of low fuel consumption, low exhaust gas emissions, good operability and high power the A/F must be kept within relatively close limits. As a rule, an A/F slightly to the lean side of the optimal power value is preferred (so-called "lean burn").
In present day high cost, sophisticated internal combustion engines, such as provided in automobiles, the problem of A/F mixture variations has been largely overcome due to the relatively recent developments in electronic fuel injection systems. Typically, such automotive fuel systems employ an electro-mechanical fuel pump delivering fuel at relatively high pressure to a solenoid-type electrically controlled and actuated fuel metering valve which is computer operated in accordance with a complex system. Many parameters of engine operation and ambient conditions are sensed continuously, and these monitored parameters are fed to an electronic processing system to control the fuel metering valve in accordance with the resultant matrix of such parameters. Again, however, the cost, complexity, bulk and reliability of such fuel injection systems is such to make the same highly impractical for use in the field of small single or dual cylinder engines such as used on chain saws, weed whips, lawn mowers, garden tractors and other small lawn, garden and forestry appliances.
In addition, small carburetors that are used in chain saws and other small engines have been decreasing in size because of the demands for smaller units to fit within all hand-held chain saws. There has also been pressure on the manufacturers of carburetors to reduce the cost of these carburetors because of the keen competition in the field. It is also desirable that servicing of the carburetors be accomplished in as expedient a manner as possible and that the number of parts in the carburetors be reduced. These factors further mitigate against use of such known prior art solutions.
Accordingly, carburetors still remain the only practical choice for gasoline fuel feeding to such small engines wherein the fuel flow to the fuel supply opening in the carburetor throat is controlled by a needle valve. Typically such carburetors are provided with a main adjustment orifice to control the main fuel supply, and an idle adjustment orifice and associated needle valve to control supply of fuel to the idle circuit located downstream of the main fuel jet in the vicinity of the throttle valve of the carburetor.
Future legal restrictions on emissions of CO will not allow manual adjustment of the carburetor. With the tolerances of manufacture of the carburetor that can be obtained it is not possible by using fixed nozzles in the carburetor to both fulfill the aforementioned legal restrictions and simultaneously assure the engine/appliance operator a good operability at all combinations of air pressure, temperature, varying fuel quality, etc. The preferred A/F is normally influenced by a number of factors. Some of these are known when the engine is designed and can therefore be corrected from the beginning, but others depend on variations of external circumstances such as air pressure, temperature, fuel quality, variations connected with the manufacture of the carburetor, and last but not least, the manner in which the carburetor engine choke and throttle controls are manually manipulated by the operator of the engine-equipped appliance.
Certain i.c. engines, such as the aforementioned automotive engines, have been provided with special oxygen sensors or lambda sondes in the exhaust system. It is thereby possible to sense engine combustion performance and the sonde measurements can be used in a self-adaptive closed loop control system to control the A/F in order to provide a good result under all conditions on a "real time" basis. However, this is an expensive and complicated control system which for reasons of cost and operational reliability can hardly be used in the aforementioned small engine consumer products such as chain saws, lawn mowers, etc.
According to present techniques used for adjusting the carburetor, the operator adjusts the carburetor manually at full throttle to obtain a recommended maximum speed of rotation. This technique is unsatisfactory to meet even the wider emission tolerances to be allowed for small engines since it does not ensure in any way that the contents of HC and CO are kept within prescribed limits. As indicated previously, products such as chain saws, lawn mowers, clearing saws, etc., require low manufacturing cost due to the low price of such consumer products. Nevertheless, due to advances and cost reductions in solid-state microcomputer electronics in the last few decades low cost solid-state magnet-type ignition systems are now customarily provided which operate without the generator or alternator of automotive systems and which provide a ready source of low wattage power and engine speed (tachometer) signals.
The availability of such solid-state ignition systems has enabled some of the foregoing problems as they relate to carburetors designed for small engines to be generally addressed by provision of an A/F control system, apparatus and method as set forth in U.S. Pat. Nos. 5,226,920; 5,284,113; and 5,345,912, which are incorporated herein by reference. As pointed out in the '920 patent, it was previously known to detect small variations in engine speed from one revolution to another by electronic means connected to a magnetic ignition system in which the signal generated by the ignition flywheel magnet in the primary or charging winding is used for measuring the speed of the engine by measuring the period of time between pulses. This method is very accurate in detecting even small speed variations and also provides a rapid response.
The '920 patent provides an A/F control system which combines the electronics of the ignition system with an electrically adjustable carburetor fuel system and comprises an electronic detector and control unit which uses a portion of the energy of the ignition magnet for feeding current to the electronic equipment so that no extra generator or battery is required. This system also includes a electronic data processing means, an electronic memory and an electro-mechanical control unit for adjusting the A/F. This adjustment is performed after a period of time during which the speed of the engine has been generally constant. The parameter used for adjustment is the first derivative of engine RPM. A basic reference value is established on the given engine measurements in the laboratory and stored in the memory of the control system.
The generally constant engine speed is detected by calculating the average value of the first derivative of the engine speed function, the speed of revolution of the engine being considered to be generally constant when the average value of the first derivative is approximately zero. The system adjusts the A/F step wise or successively when the engine is operating under load until the first derivative of speed variations has reached a predetermined level, or a break point of lean adjustment as detected as a function of a reduction of the speed revolution of the engine. If the measured discrete absolute value of the first derivative of engine speed variation when averaged exceeds a reference value measured in the laboratory, the system determines that the air/fuel mixture is too lean. The A/F mixture is then adjusted richer in steps of about 4% until the average value of the first derivative is close to the reference value. In this manner the air/fuel ratio of the engine is adjusted with regard to a previously known speed dependency of the A/F to provide a modified speed dependency of the ratio preferably approaching a constant A/F over the operational speed range of the engine. Although the '920 patent states generally that the adjustment to A/F is carried out by a micro computer which controls drive circuits of an electric motor connected to the fuel nozzle of the carburetor of the engine whereby various adjustments can be made to the fuel nozzle by the computer, no such fuel nozzle control mechanism is otherwise shown or described.
The '912 patent adds a second A/F control system and means for adjusting the A/F based on actual operating conditions using a feedback system which takes all variations into account affecting the A/F at the time of sensing. A fuel needle is provided for A/F testing which is actuated between open and closed positions by a solenoid valve to thereby open and close a secondary or by-pass flow path to the main fuel nozzle of the carburetor. When this by-pass path is closed the secondary flow is shut-off while the primary flow continues, thereby reducing total fuel flow to the main fuel nozzle and thus changing the A/F to a leaner mixture for a short period of time. The change of speed of rotation on the engine occurring in response to this leaning of the A/F mixture is measured to determine whether the A/F in existence prior to the shut-off adjustment test is a leaner or richer mixture compared to a preferred level or optimum point in the engine power curve. The A/F is then adjusted by a predetermined step towards the preferred level by actuating an A/F adjusting means, such as by modulating the air pressure differentials acting on the diaphragm of the carburetor. It is to be noted that the '912 patent also suggests that, instead of controlling the reference pressure in the diaphragm air chamber, one or two fuel nozzles could be controlled by a throttle needle in the main throttle flow and controlled proportionately by an electric motor. However, such an alternative is neither shown nor further described.
The test procedure is repeated by the second control circuit until the change of engine RPM indicates that the mixture ratio is at the preferred level. This adjustment is then maintained for a period of time after which the second or test control circuit resumes the testing and adjustment of the mixture ratio.
The periodical testing in which the solenoid needle is actuated to close the secondary fuel feed to temporarily lean out the mixture must be of as short a duration as possible so that the engine user is generally unaware of the test procedure taking place. This test control circuit can thereby provide A/F correction for a plurality of disturbances to which the engine might be exposed, such as variations of air pressure and temperature, fuel type and quality as well as defects in the manufacture of the carburetor such as tolerance variations.
Other strategies for adjusting and/or controlling an electrically adjustable carburetor are set forth in the published European patent application Publication No. 0 297 670 A2 published Jan. 4, 1989 and in U.S. Pat. Nos. 4,617,892; 4,949,692 and 5,284,113. In the first three of these approaches, the absolute value of engine speed is utilized rather than the first derivative of speed variations of the engine. In EPA 0 297 670 A2, a control unit develops a control signal for a stepper type motor having a pinion gear on its output shaft engaging a rack teeth on a rod to rotate a threaded fuel needle to thereby control flow through a fuel nozzle to thereby vary the air/fuel ratio delivered by the carburetor to the engine.
As shown semi-schematically and briefly described in U.S. Pat. No. 5,284,113, an externally mounted electric motor 16 rotates an angled gear 17 on a shaft 14 threadably engaged in a carburetor housing bore 18 and having a fuel flow adjusting needle 12 at its inner end, which can be made as a self-braking screw to maintain the adjusted needle setting when the engine is shut off. However, no disclosure is provided as to how such a needle drive is to be constructed and integrated into the carburetor structure in a practical manner. In U.S. Pat. No. 4,617,892 no fuel flow controlling devices are shown and are merely stated generally to be a fuel injection system or a carburetor with electrically controllable metering. U.S. Pat. No. 4,949,692 references generally an electronic fuel metering valve not otherwise shown or described, or an electrical flow controller (EFC) such as that manufactured by Borg-Warner Corporation, U.S.A. said to operate as a variable orifice which responds to a digital pulse width modulated electrical signal at a fixed frequency. Again, only a schematic showing is provided without any disclosure of how such adjustable fuel flow devices are to be constructed and/or manufactured nor economically integrated into a small engine diaphragm carburetor in a practical manner.
Although the method of controlling a carburetor with electrically controllable metering in accordance with the previously discussed '912 patent has been deemed to be one preferable approach with respect to the problems associated with controlling A/F ratio in small engine diaphragm carburetors, attempts to implement this method in practical devices has led to the discovery of several additional problems needing solution. In order to obtain simplicity and reduce costs in this type of carburetor it would be desirable to eliminate, for regulation of the A/F mixture, the added external equipment associated with the vacuum pump and vacuum line connected to the air chamber of the diaphragm carburetor as set forth in the '912 patent. Also limitations of the '912 patent "test-adjust-repeat" fuel control mechanism also inherently makes it difficult to shorten the duration of each lean out phase of the test cycle so as to minimize interruption of the normal engine operating mode. It has been found that shutting off fuel flow through a second or tributary fuel path between the diaphragm fuel metering chamber and the main jet or nozzle does not adequately satisfy the need for a precisely controlled and short lean out time.
Additionally, the working environment for diaphragm carburetors on small engines subjects the carburetor and the automatic control components to severe vibrations, engine heat, rough handling and other adverse working conditions. These environmental conditions render reliable and repetitive automatic control mechanisms difficult to achieve in a practical and economical manner, particularly when attempting to finely adjust the A/F over a small range to optimize the proper combination of low fuel consumption, low exhaust gas emissions, good operability and high power.
Moreover, in order to adapt such electrical adjustment systems to conventional diaphragm carburetors for small engines, it is necessary to retain the conventional butterfly choke valve and idle fuel feed system customarily provided in such carburetors. This poses additional problems in attempting to implement the periodic test-lean-out-adjust control system of the '912 patent in practice. In conventional non-electrical small engine carburetors utilizing a flexible diaphragm for regulation of fuel flow to both the main and idling nozzles or orifices, when the engine is operating at wide open throttle the fuel bleeds or is removed from the idle circuit of the carburetor. Consequently, when the engine goes from full throttle to idle, it frequently stumbles and sometimes stalls because the idle circuit then supplies insufficient fuel to the engine. If the carburetor is automatically controlled such that the A/F is maintained on the lean-burn side, such as according to the automatic control system of the '912 patent, it has been found that this stumble and stall problem is aggravated because the mixture is already on the lean side. Furthermore, when operating at part throttle, the carburetor tends to supply a fuel mixture which is richer than the idler mixture for operation of the engine due to an adverse influence of continued fuel supply from the idle circuit of the carburetor, and which is not subject to automatic A/F control.
One solution to such problems as embodied in a non-electrical carburetor is set forth in U.S. Pat. No. 5,250,233 issued in the name of Mark S. Swanson and assigned to the assignee of record herein, which is incorporated herein by reference. In this invention a combination accelerator pump and shut off device is provided to control the fuel to the idle chamber. Preferably, the accelerator and shut off device is actuated by movement of the throttle from its idle position to initially supply a relatively small quantity of additional fuel for accelerating the engine, and to shut off the idle circuit under wide open throttle operating conditions. This prevents bleed back of fuel in the idle circuit so that when the throttle returns to its idle position and the shut off device opens, idle fuel remains available and thus is immediately supplied to the idle jet for operation of the engine under idle conditions. Moreover due to the "lost motion" between the piston and valve of the '233 patent mechanism, when the throttle is only partially opened the supply of fuel to the idle well and associated idle ports is shut off, thereby eliminating the influence of the idle circuit on the A/F mixture under engine partial load conditions so that the fuel mixture is determined solely by the main nozzle of the carburetor. It has been found that such problems as disclosed in the '233 patent with respect to a conventional non-electrical diaphragm carburetor are also present and even more severely impair electrically controlled fuel metering to optimize the A/F, particularly when utilizing the method and devices of the aforementioned '920 and '912 patents inasmuch as they control A/F adjustment by modulating fuel flow only to the main nozzle.
Additional problems in implementing electrically controllable metering in a small engine diaphragm carburetor have been found to arise from the need to retain manual control of both the choke and throttle valves, thereby allowing the uncontrollable variable of operator manual intervention in the control system to defeat the automatic system goals.
Of course, there are also the overriding problems associated with attempting incorporate the automatic system components into a low cost and compact carburetor package without unduly complicating the component design, increasing the cost of manufacture and assembly of the carburetor and sacrificing operational life and reliability as well as serviceability.